Abstract: Ni₄₇Ti₄₄Nb₉ shape memory alloy (SMA) is a promising material in the aerospace field due to its wide transformation hysteresis. The application of shape memory effect depends on multistep thermomechanical loading, viz., low-temperature deformation and subsequent heating to recovery. Low-temperature deformation prestrain plays a pivotal role in shape memory properties tailoring of SMA components. However, microstructure evolution and deformation mechanisms of Ni₄₇Ti₄₄Nb₉ SMA subjected to various prestrain levels are still unclear. To this end, microstructure evolution and shape memory behaviors of Ni₄₇Ti₄₄Nb₉ alloy subjected to multistep thermomechanical loading with prestrain levels of 8% - 16% at -28 °C (M_s + 30 °C) were investigated. The results demonstrate that the stress-strain curve of the specimen exhibits four distinct stages at a maximal prestrain of 16%. Whereas stage II and stage III end at prestrains of ∼8% and ∼12%, respectively. In stage II, the stress-induced martensitic transformation is accompanied by the dislocation slip of the NiTi matrix and β-Nb inclusions. In stage III, in addition to the higher density of dislocations and further growth of stress-induced martensite variants (SIMVs), (0 0 1) compound twins are introduced as a result of the (0 0 1) deformation twinning in stress-induced martensite. More {2 0 -1} martensite twins are gradually introduced in stage IV. Correspondingly, after subsequent unloading and heating, a higher density of {1 1 4} austenite twins form in the specimen with a larger prestrain of 16%. With increasing prestrain from 8% to 16%, the recoverable strain $ \varepsilon_{\mathrm{re}}^{\mathrm{T}}$ upon heating increases first and then decreases. The $ \varepsilon_{\mathrm{re}}^{\mathrm{T}}$ obtains a maximum of 7.03% at 10% prestrain and decreases to 6.17% at 16% prestrain. The increase of $ \varepsilon_{\mathrm{re}}^{\mathrm{T}}$ can be attributed to the formation of new SIMVs, the further growth of existing SIMVs, and the recoverable (0 0 1) compound twins. While the decrease of $ \varepsilon_{\mathrm{re}}^{\mathrm{T}}$ is mainly associated with the irrecoverable strain by {2 0 -1} martensite twins. The effect of β-Nb inclusions on the evolution of SIMVs is also found herein that deformed β-Nb inclusions can significantly hinder the growth and recoverability of adjacent stress-induced martensite.

Key words: Ni₄₇Ti₄₄Nb₉ shape memory alloy, Wide transformation hysteresis, Thermomechanical loading, Microstructure evolution, Shape memory behaviors, Stress-induced martensitic transformation, Deformation twinning